This invention relates generally to meltblowing apparatus and processes. In one aspect, it relates to a meltblowing die consisting of side-by-side units for meltblowing polymer filaments, each unit including metering means to control the flow of polymer melt therethrough.
Meltblowing is a process for forming nonwoven webs by extruding a polymer melt through a plurality of orifices while contacting the extruded filaments with hot air to drawdown and attenuate the filaments into microsize fibers. The fibers are collected in random on a collector such as a rotary screen or surface forming an entangled fibrous web. The microfibers in the web resulting from the extreme drawdown impart unique properties to the web, making them ideally suited for several applications including filters, oil wipes, battery separators, oil absorbers, layers of web within diapers, and other absorbent products, etc.
The meltblowing die must be carefully designed and machined in order to provide the web of the quality (in terms of uniformity in basis weight and thickness) for many of the above industrial applications. The several variables in the process such as polymer throughput, air flow rate, collector take-up rate, and the like must be carefully controlled to provide a web with generally uniform properties for many applications. For wide webs, this sometimes is a problem because of the difficulty in achieving uniform polymer flow and air flow throughout the length of the die. Uniform flow of the melt through the die is normally achieved by the use of a balancing flow channel. (Sometimes referred to as a coat hanger die or flow channel) such as that disclosed in U.S. Pat. No. 4,818,463. This channel is carefully machined to provide a uniform distribution of the polymer to the orifices. In order to provide a uniform air flow across the die, the die assembly sometimes is provided with diverters in the flow channel and means for uniformly distributing the air across the air inlets to the die, as disclosed in U.S. Pat. No. 4,818,463.
The dies disclosed in the above U.S. Patent, as well as other commercial meltblowing dies, generally have one polymer inlet through which the polymer melt flows into the coat hanger channel. For small dies, the aforementioned coat hanger balancing channel is satisfactory, particularly when coupled with the uniformly controlled airflow. However, for medium to long dies, the polymer flow from the single polymer inlet to the orifices is long, particularly at the outer ends of the die. This introduces variations in melt flow through the orifices and adversely affects the quality of the web, particularly the uniformity of web basis weight and thickness.